1. Field of the Invention
The present invention relates to an image processing apparatus and image processing method, and more particularly to a technique for generating data for completing printing in the same area of a printing medium by moving a printing head a plurality of times relative to the same area of the printing medium.
2. Description of the Related Art
As this kind of technique, a multi-pass printing method is known for completing an image to be printed in the same area of a printing medium by scanning that same area of the printing medium a plurality of times with a printing head. With this technique, it is possible to reduce density unevenness and stripes in an image. However, even though the multi-pass printing method is employed, there is a possibility that a shift of the dot printing position will occur between the plurality printing scans due to fluctuation in the conveyance amount of the printing medium. Such shift causes fluctuation in the dot coverage, and as a result, image defects such as density fluctuation or density unevenness may occur.
In addition, the fluctuation in the conveyance amount of the printing medium mentioned above differs according to the position of the printing medium on the conveying path of the printing medium. For example, when printing over the entire surface of a printing medium without setting margins, or in other words, performing so-called margin-less printing, the printing position is located on the printing medium where printing is performed by the printing head as the printing medium is conveyed along the conveying path by only one pair of conveying rollers that are provided on both the upstream side and downstream side of the printing area. In such a printing position, the conveyance precision is low and it is easy for fluctuation in the conveyance amount to occur. On the other hand, when the printing medium is conveyed in a state of being held by both of a pair of conveying rollers on the upstream side and downstream side, the conveyance precision is high and it is difficult for fluctuation of the conveyance amount to occur. Moreover, it is known that during conveyance, when the printing medium enters or leaves the nip section of a pair of conveying rollers, there is a relatively large fluctuation in the conveyance amount.
A method is known as a technique for reducing the image defects caused by the shift of the dot printing position due to the above described fluctuation of the conveyance amount in which image data are divided into respective data corresponding to different scans in the stage of multi-valued image data before binarization, and then the respective divided multi-valued image data are independently binarized (Japanese Patent Laid-Open No. 2000-103088). FIG. 15A is a diagram illustrating the arrangement of dots that are printed based on image data that are processed by the method disclosed in Japanese Patent Laid-Open No. 2000-103088. In the figure, the black dots 551 are dots that are printed in a first printing scan, white dots 552 are dots that are printed in a second printing scan and gray dots 553 are dots that are printed by overlapping of dots that are printed in the first printing scan and second printing scan. In other words, the technique disclosed in Japanese Patent Laid-Open No. 2000-103088 is a technique of generating overlapping dots by reducing mutual complementarity or exclusivity of dots that are printed in a plurality of printing scans.
With this dot arrangement, even though a dot group printed in a first printing scan and a dot group printed in a second printing scan become shifted in the main scanning direction or sub scanning direction, there is not much fluctuation in the dot coverage on the printing medium, and as a result, it is possible to reduce the image defects mentioned above. This is because even though portions, where dots that are printed in the first printing scan and dots that are printed in the second printing scan overlap, newly appear, there are portions where two dots that were originally supposed to be printed such that they overlap do not overlap. More specifically, conventionally, by using a mask on normally quantized data, print data are divided, as well as complementarity and exclusivity is given to data that are to be printed indifferent printing scans. On the other hand, the method disclosed in Japanese Patent Laid-Open No. 2000-103088 is a method in which multi-valued data are divided in the multi-valued data stage into a plurality of multi-valued data that correspond to different printing scans, and the plurality of multi-valued data are each independently quantized so that quantized data to be used in the respective printing scans are obtained. This allows complementarity between dots that are printed in different printing scans to be decreased, and overlapping dots are caused to occur among the dots that are printed in a plurality of printing scans.
However, in the method disclosed in Japanese Patent Laid-Open No. 2000-103088 it is not possible to control the amount of dot overlap that are printed in a plurality of printing scans. As a result, the number of overlapping dots may become excessive and the graininess of the image may be increased, or conversely, the number of overlapping dots could be too few and the aforementioned density fluctuation may not be sufficiently suppressed.
The inventors of the present application placing attention on making it possible to control the amount of the dot overlap by generating data that is reflected in all the quantized data of the different printing scans in common with each other.
On the other hand, the degree of graininess or density unevenness (density fluctuation) described above that finally appears in the printed image differs according to the color used for printing. For example, in the case of a color having high lightness such as when the printed color is yellow (Y), the degree of graininess or density unevenness does not change so much regardless of the amount of dot overlap. In contrast, in the case of a color having low lightness such as when the printed color is black (K), change in the degree of graininess or density unevenness (density fluctuation) due to the amount of dot overlap is comparatively large.
Moreover, there may be cases in which bleeding at the boundaries between different colors of ink (hereafter, simply referred to as bleeding) depending on the amount of dot overlap becomes worse. Particularly, when using pigment BK ink and dye Y ink, which permeate into the printing medium at different permeation speeds, it becomes easy for the bleeding to be highly visible. Therefore the amount of dot overlap becomes an important factor for decreasing the bleeding.
When the degree of graininess or density unevenness that appears in a printed image differs according to colors as described above, or when the amount of bleeding that occurs at the boundaries between different colors differs according to inks used, applying a process of generating data that commonly reflects on all of the data is not preferred. More specifically, when applying a process of generating data that commonly reflects on all data, there may be cases in which graininess and the like become just worse. In addition, by performing such processing when generating data for colors for which the processing is not so necessary, there may be an increase in the processing load or a decrease in the processing speed.